Filler-primer

[bob888]

I am ready for a filler primer to be applied on the windshield fairing and some other areas of fiberglass buildup around the door frame. Any recommendations favoring a specific product? How are these applied - brush, roller, spray?

 

[Carl Froehlich]

I tried Smooth Prime on the fiberglass for the RV-10 because of a strong recommendation from a Lancair 4 builder. I will never use it again (many reasons).

Sand, do a skim coat of micro balloons, sand again. Repeat if needed. At this point I just add the same primer I'm using for top coat (PPG DP-40LF). For the next plane I'll use PPG K36 high fill primer, wet sand and then do the DP-40LF on top of that when ready to do final paint.

Carl

 

[AviatorJ]

What were some of your issues with the smooth prime? I had planes to use that, not spray it but wipe it on and skim coat it.

On my empennage pieces I used those as a test bed and found epoxy skim costs (2 or 3) really too care of all pinholes.

 

[DanH]

I am ready for a filler primer to be applied on the windshield fairing and some other areas of fiberglass buildup around the door frame. Any recommendations favoring a specific product? How are these applied - brush, roller, spray?

Bob, you're gonna get a lot of different opinions. Walk around enough fly-ins looking at unpainted RV's, and you'll see that quite a few builders tried every magic filler product on the market.

Ok, here's the conservative approach. Microballoons and epoxy are mixed to a firm paste and squeegeed on as necessary, then sanded for contour. The surface is sealed with a skim coat of straight epoxy to fill the pinholes, then sanded dull without breaking through. Epoxy primer is followed by high build primer, both from the same manufacturer's approved system.

Contouring with micro; pencil marks highlight low spots, which will be filled/sanded again. Just scribble with an ordinary #2, then block sand. The pencil marks sand off the highs and remain in the low spots.



Countoured part being sealed with an epoxy skim coat. A few years back I started rolling the wet squeegeed surface with a cheap nappy roller, as it eliminates all the squeegee marks. It also leaves a nice "orange peel" sort of finish on the epoxy, which makes it easy to sand slick without breaking through...you can easily see how much you've cut.



Looks like this before cure and sanding:



Last, shoot with epoxy primer. Countouring was good enough to make a high-build primer unnecessary.

At this point I just add the same primer I'm using for top coat (PPG DP-40LF). For the next plane I'll use PPG K36 high fill primer, wet sand and then do the DP-40LF on top of that when ready to do final paint.

Hi Carl. For maximum adhesion of K36 (and subsequent top coats) on a composite surface, shoot DPLF epoxy primer, then K36 as soon as the solvent flashes off. The PPG field rep's actual words were "...as soon as you can clean and reload the gun".

You can shoot more DPLF over block-sanded K36 prior to topcoats, but it's not strictly necessary. I don't because a sprayed surface finish isn't likely to be as slick as a block-sanded surface...and imperfection will show through the topcoat.

 

[Carl Froehlich]

What were some of your issues with the smooth prime? I had planes to use that, not spray it but wipe it on and skim coat it.

On my empennage pieces I used those as a test bed and found epoxy skim costs (2 or 3) really too care of all pinholes.

If you do use Smooth Prime, thin it with just a touch of water and spray it. You will save yourself days of sanding. I first tried the roller/skim coat approach and quickly regretted it. Using just enough water so an HVLP sprayer worked significantly reduced sanding time.

If you do use Smooth Prime NEVER wet sand. If the stuff gets wet you need to let it dry out for a few weeks before applying your primer for top coat. Even so, you have to let the stuff cure a long time before putting a primer on top. You will immediately know if you did not wait long enough as the primer will bubble as soon as it hits the Smooth Prime.

Overall, much less work to just follow Van's recommendations, prime, wet sand and prime again if needed. Looking over other projects that followed Van's instructions then added a coat of K36 (or equivalent) and wet sanded that had a lot less work than I did.

Carl

 

[9GT]

I know a guy who used Smooth Prime as his base primer on his Cozy MKIV years ago. After about a year it started peeling. He ended up scraping and sanding everything back down to bare glass and repainted using traditional epoxy primer, high build primer, and urethane top coat. I have read other similiar stories on the product in its early days. Perhaps the formula has improved since or maybe users did not follow directions properly?

 

[Kyle Boatright]

I know a guy who used Smooth Prime as his base primer on his Cozy MKIV years ago. After about a year it started peeling. He ended up scraping and sanding everything back down to bare glass and repainted using traditional epoxy primer, high build primer, and urethane top coat. I have read other similiar stories on the product in its early days. Perhaps the formula has improved since or maybe users did not follow directions properly?

One of the things they discovered was that you have to wait until it is *really* dried (cured?) before you paint over it. Here's what it says on the Aircraft Spruce website:

Let it dry for 3 weeks. If you live in a cold, damp climate invest in a dehumidifier~ buy or rent it. ALL WATER must be out of the UV Smooth Prime before it is over-coated. Next, you will need to seal the UV Smooth Prime with a 2-Part Epoxy Primer....

I know of people <ahem> who didn't follow that procedure (maybe because it wasn't published at the time?) and who have seen problems down the road.